Remove dependencies on Boost.StaticAssert.

Boost.StaticAssert has been merged into Boost.Config, so remove
the dependency.

Author: Lastique

.ci/get-boost.sh

@@ -72,7 +72,6 @@ git submodule --quiet update --init $GIT_SUBMODULE_OPTS \
     libs/ratio \
     libs/rational \
     libs/regex \
-    libs/static_assert \
     libs/smart_ptr \
     libs/system \
     libs/throw_exception \